1. Field of the Invention
The present application relates to insulation methods and systems and more particularly, but not by way of limitation, to methods and systems for internal convective insulation of hollow sill components associated with, for example, doors and windows.
2. History of the Related Art
The trend of increasing prices for natural gas, electricity, and other heating fuels has made energy efficiency a high-profile issue. In buildings, thermal energy may be lost to the atmosphere through, for example, conduction, radiation, or convection. Conduction is a transfer of thermal energy between regions of matter due to a temperature gradient. Radiation is a transfer of thermal energy via electromagnetic waves. Convection takes place as a result of molecular movement, known as currents or convective looping, within fluids. A common mode of convection results from an inverse relationship between a fluid's density and temperature. Heating of a fluid results in a decrease in the fluid's density. Denser portions of the fluid fall while less dense portions of the fluid rise resulting in bulk fluid movement. Typically, such type of convection is referred to as “natural” or “free” convection. A common example of natural convection is a pot of boiling water in which hot (less dense) water at a bottom of the pot rises in plumes and cooler (more dense) water near a top of the pot sinks. The primary means of thermal energy loss across an un-insulated air-filled space is natural convection.
Thermal efficiency of building components is often expressed in terms of thermal resistance (“R-value”) and thermal transmission (“U-factor”). R-value, a measurement of thermal conductivity, measures a product's resistance to thermal energy loss. In common usage, R-value is used to rate building materials that generally do not transfer significant amounts of thermal energy by convection or radiation such as, for example, insulation, walls, ceilings, and roofs. A product with a higher R-value is generally considered to be more energy efficient.
In building insulation, of particular concern are windows and doors. Windows, in particular, come into contact with the environment in ways that walls and solid insulation do not. As a result, windows are strongly affected by convection as well as radiation. For this reason, U-factor is commonly used as a measure of energy efficiency of windows. U-factor measures a total rate of heat transfer through a product (including heat transfer via convection and radiation). A product with a lower U-factor is generally considered to be more energy efficient. In recent years, federal, state, and municipal building codes often specify minimum R-values and U-factors for building components.
Door and window assemblies of most buildings typically include one or more sill members. In most buildings, the sill members may be constructed from, for example, extruded materials having a hollow cavity therein. In moderate conditions, air contained in the hollow cavity often provides sufficient insulation to prevent thermal energy loss through the sill members via, for example, conduction, convection, or radiation. However, in conditions where there is a large temperature difference between an interior temperature and an exterior temperature, the large temperature difference may induce thermal currents in air contained in the hollow cavity making thermal energy loss via convection through the sill members significant.
In window and door assemblies, solid insulation such as, for example, foam or fiberglass has been used to reduce thermal energy loss through conduction and convection. However, solid insulation is not well suited for use in exterior door and window sill applications for a variety of reasons. First, installation of solid insulation throughout a sill member may prove difficult and time consuming. Second, infiltration of moisture into solid insulation materials often fosters growth of, for example, bacteria, fungus, and other contaminants. Such contaminants can cause unpleasant odors, aggravate allergies, and cause illness.